DISPERSION OF POLLEN BY ANIMALS. 165 become flaccid, midges that have been imprisoned at the bottom of the cavity pass into the upper story, they are kept there for a time by the upper bristles, which are still rigid, so that the insects knock against the male flowers and must cover themselves with pollen. Finally, when this object is achieved, the upper bristles also relax and the midges are allowed to escape. It is astonishing what a large number of insects and what a variety of different kinds find a home in the fiowers of Aroideae. The smaller Aroids, such as Arum maculatum, widely distributed in Europe, are sought chiefiy by tiny midges of the species Psychoda phallcenoides, and it is not uncommon to find several hundreds of them in the cavity of a single spathe. In the receptacle formed by the spathe of an Arum conoeephaioides, planted in the Botanic Gardens of Vienna, three species of small black midges of the genus Geratopogon had congregated, and were present in such large numbers that when one of the spathes was opened artificially a whole swarm fiew out. A second spathe of the same plant, which was immersed in alcohol and subsequently opened, was found to contain nearly a thousand midges of the kind. In the Italian Arum {Arum Italicum) also as many as sixteen different species of files, mostly of the genera Chironomus, Limosina, Sciara, and Psychoda, have been found in a single spathe. Another Aroid, Dracunculus crinitus, is sought principally by large flies belonging to the species named Somomyia Caesar and Anthomyia scalaris. In the receptacles formed by the spathes of the Dracunculus Creticus, which has flowered in the Botanic Gardens of Vienna, various carrion- beetles {Aleochara fuscipes, Dermestes undulatus, Saprinus nitidulus, &c.) had collected, besides numerous green-gilded flies of the genera Anthomyia, Lucilia, and Somomyia; and in the shea thing-bracts of Dracunculus vulgaris which grows in Italy scarcely anything but carrion-beetles of the genera Dermestes and Saprinus have been observed. A single spathe of the last-named plant was once found to contain more than 260 carrion-beetles belonging to eleven different species. The flowers of the Birthwort genus {Aristolochia) bear a surprising resemblance to the spathes of Aroideae, their perianths being, like aroid spathes, divided into three regions. First of all, there is the limb, which in the European species has the form of a trumpet, and in the tropical species of America assumes many other curious shapes, as, for instance, that exhibited by Aristolochia ringens (fig. 242), where it is drawn out into a boat-shaped under-lip with an upper-lip arching over it. Next comes a tubular median portion, which is furnished with various contrivances to prevent the egress whilst permitting the entrance of creatures seeking shelter. Lastly, there is an enlarged basal portion like a bladder or pouch wherein the stigma and anthers are situated, and which constitutes the goal of the insect-visitors. On a future occasion it will be necessary to enter more fully into the manner in which the insects that creep into the pouch take up and afterwards deposit the pollen, and it will therefore be sufficient to mention here that they are kept pri¬ soners there until the anthers have opened. When dehiscence has taken place, and not before, the tubular middle region undergoes certain changes which make it possible for the captives to escape from their temporary dungeon.